The invention relates to a continuously and automatically functioning device for the drainage of sludge treated with a flocculent, especially of sewage sludge, with a screw conveyor installation with a driven conveyor screw consisting of shaft and helix and a screen wall. Instead of the conveyor screw also the screen wall may be driven in a rotating way and the conveyor screw be stationary. Especially in sewage engineering sludges of the most diverse kinds can be encountered, for example crude sludge, digested sludge, return sludge, excess sludge, scum, etc. which possess a very high water content and a correspondingly low solid-matter content. The automatically functioning device serves the purpose of increasing the solid-matter content of sludge thickened by means of a flocculent. It can be used in sewage engineering, but also in the paper industry and in the manufacture of textiles.
A device of the kind mentioned at the outset is known. The sludge treated with a flocculent is transported to a receptacle in which a stirring shaft with a vertical axis is located. In its lower section the receptacle is conical and is followed by a screw conveyor installation with a horizontal axis, which is equipped with a cylindrical screen wall and possesses a helix by means of which the sludge is drained and compressed. The end of the screw conveyor installation is more or less locked by means of a flap to which a balancing weight may be attached in order to generate a pressure build-up in the interior of the screw conveyor installation. In the area of the screen wall the filtration water is drawn off. The generation of pressure in the screw conveyor installation is only in part suitable for the separation of water. Since the solid-matter quantity contained in the sludge is often of a very fine consistency there exists the hazard that a pressure increase causes the solid particles to be pressed through by way of the openings of the screen wall, while the desired water separation will then not take place anymore. It also happens that the pressure build-up leads to a compression of the solid matter quantity in the range of the screen wall, while in the interior of the screw conveyor installation in the range of the shaft the sludge continues to be relatively moist because in this place the water cannot penetrate to the outside through the compressed solid matter.
Another known device for the drainage of sewage sludge provides a screen zone with a subsequent pressing zone, a jointly driven conveyor screw which consists of shaft and helix extending through these two zones. In the screen zone the housing wall of the screw conveyor installation is designed as screen, while in the pressing zone it is closed. Also in this case the housing is closed to a large degree in the area of the solid-matter delivery chute through the forming of clots so that in this way there will be a back-draft of the solid-matter in the screw conveyor installation. By means of this back-draft a pressure build-up is achieved at the same time.
Moreover, belt presses for the drainage of sewage sludge are known. They are advantageously continuous. However, on the belts a filter cake builds up from the solid matter contents which renders the further penetration of filtration water more difficult. By using such belt presses solid-matter contents of up to 25% can be achieved. While known chamber filter presses are not continuous they render possible higher solid matter contents of up to approximately 36%. The filter cakes possess different degrees of moisture from the core to the outer areas, the interior being the most humid zone.